Broadcasting apparatuses, microwave ovens and the like require high power microwaves. Usually, such microwaves are obtained by using a magnetron apparatus, and such a magnetron is installed within a shielding metal case in order to prevent the leaking of radio waves, with a through type condenser being used in supplying an accelerating voltage to the magnetron. The accelerating voltage which is supplied through the through type condenser is a very high voltage of about 4.2 KV, and therefore, the condenser has to be manufactured after taking into account its voltage resistent characteristics. Particularly, the microwaves which are generated from the magnetron after receipt of the accelerating voltage have a high frequency amounting to about 2.45 GHz in its primary wave. Further, there is a high possibility that secondary and tertiary harmonic waves can be leaked through the open-through type condenser to the outside, and therefore, a particular attention has to be given in manufacturing the through type condenser.
FIG. 1 illustrates the circuit of a magnetron, and, as shown in this drawing, a magnetron M1 is installed within a shielding case S1. Further, an accelerating voltage is supplied to a grounded positive electrode A1 and a heater H1 of the magnetron M1 through condensers C1,C2 which form an LC resonant circuit. Therefore, if a magnetic field is supplied in a direction perpendicular to the ground surface, the electrons emitted from the cathode arrive at the anode in a rotating manner. Under this condition, several electronic electrodes are formed due to the space charge. In accordance with the rotation of the electronic electrodes, microwaves are generated from the resonant circuit of the positive electrode.
In the field of this through type condenser technology, Nikio et al proposed a through type condenser having high voltage resistent characteristics and this invention has the title of "Through Type High-Withstand-Voltage Ceramic" and is disclosed in U.S. Pat. No. 4,370,698. As shown in FIG. 2, the through type condenser of Nikio et al includes: a ceramic body 14 having a pair of through-holes 12,13; a pair of conductive rods 10,11 passing through the through-holes 12,13; and an insulating resin member 16 completely surrounding the ceramic body 14 to form an outer casing. Here the elliptical ceramic body 14 serves as a dielectric medium for the condenser. However, the through type condenser as described above requires a large number of component members and an expensive ceramic body, and therefore, the productivity is lowered, as well as increasing the product unit price.
In an attempt to get rid of the above described disadvantages of the dielectric type through-condenser, a technology for manufacturing the through type condenser by means of a single resin member is developed as disclosed in Japanese Utility Model Application Laid-Open No. Sho-68-89664. As shown in FIGS. 3 and 4, this apparatus includes: a conductive core member 20 having a U-shaped main body 24 and a flat outer connecting terminal 22; a high potential inner electrode 26 having a guide slot 30 at its bottom 28 (cup-shaped and made of a metal) in order to guide the conductive core member 20; a cylindrical ground potential electrode 32 having a flange 36; and a single insulating resin member 38. Thus, this apparatus provides a through condenser which can be manufactured in a relatively simple manner.
Particularly, in this technology, the outer casing of the condenser and the insulating resin member which serves as a dielectric medium for the condenser are formed on a straight line. Therefore, when molding the resin, the flow of the resin can be promoted, with the result that the generation of inner pores can be inhibited, thereby improving the voltage resistent characteristics.
However, the apparatus of Japanese Utility Model Application Laid-Open No. Sho-68-89664 is encountered with many difficulties in its manufacturing process. First, the body of the conductive core member 20 has to be bent into a U-shape, and this not only lowers the productivity but also increases the number of the steps of the manufacturing process. Further, the cylindrical high potential inner electrode 26 has a bottom face 28, and therefore, it can not be manufactured by cutting an ordinary pipe, but it has to be manufactured as a separate unit component. Particularly, the bottom face 28 has to be provided with a guide slot 30 for passing the U-shaped body portion 24 of the conductive core member 20, thereby also increasing the number of the manufacturing steps.
Further, although the ground potential electrode 32 is simply cylindrical, one end of it has to be provided with a rectangular flange 36 in an integral form, and therefore, a high die technology and a high processing technology are required. Particularly, the flange 36 can not be other than rectangular, because two finished through-condensers are joined together by means of their flanges to form a pair, after the through-condensers are separately manufactured. In addition to the difficulties encountered in manufacturing the individual components, there are also encountered difficulties in assembling them. The cylindrical high potential electrode 26 and the conductive core member 20 have to be aligned exactly concentrically, and the high potential electrode 26 has to be also aligned with the ground potential electrode exactly concentrically. Therefore, special jigs are required in assembling them, and the electrical characteristics of the condensers can be deviated due to the errors existing in the jigs.